1. Field of the Invention
The present invention relates to a method and device for reducing the temperature in a hotter running cylinder of a multi-cylinder marine engine.
2. Description of Related Art
Personal watercrafts have become popular in recent years. This type of watercraft is quite sporting in nature and is designed to carry a rider and possibly one or two passengers. A relatively small hull of the personal watercraft commonly defines a rider""s area above an engine compartment.
An internal combustion engine frequently powers a jet propulsion unit which propels the watercraft. The engine lies within the engine compartment in front of a tunnel formed on the underside of the watercraft hull. The jet propulsion unit is located within the tunnel and is driven by a drive shaft. The drive shaft commonly extends between the engine and the jet propulsion device, through a wall of the hull that forms a front gullet portion of the tunnel.
Personal watercrafts often employ an in-line, multi-cylinder, crankcase compression, two-cycle engine, usually including two or three cylinders. The engine conventionally lies within the engine compartment with the in-line cylinders aligned along a longitudinal axis of watercraft hull (in the bow-stem direction).
An exhaust manifold typically couples the exhaust ports of the engine cylinders to an exhaust system. The exhaust manifold usually includes several of runners. Each runner communicates with an exhaust port of one of the engine cylinders. The runners merge together at a downstream point and communicate with an exhaust pipe of the exhaust system at a common end. The length of the runners commonly differ in prior exhaust manifolds.
The exhaust system discharges exhaust byproducts from the watercraft. The exhaust system commonly includes a water jacket which cools at least a portion of the exhaust system. At least a portion of the cooling water usually is introduced into the exhaust stream after an expansion chamber of the exhaust system to further silence exhaust noise and for discharge from the watercraft.
One of the cylinders often runs hotter than the other cylinders in prior marine engines. For instance, in a two cylinder engine, the first cylinder usually runs hotter than the second cylinder. This occurs in part because of exhaust gas flow differences between the cylinders. The exhaust flow differences between the cylinders is largely attributable to the differences in length and shape of the exhaust manifold runners.
The first cylinder, which exhausts into the exhaust manifold upstream of the second cylinder, usually has better scavenging. As a result, more complete combustion occurs first cylinder than in the second cylinder. The first cylinder thus produces more power and heat. A temperature difference consequently results between the cylinders.
Elevated operating temperature of one cylinder commonly causes the engine to knock. The high temperature also tends to deteriorate rubber products which are located near the engine or exhaust manifold. For example, vibration-attenuating rubber engine mounts, which are usually located near the exhaust manifold, may deteriorate due to high operating temperature of the engine and the exhaust manifold.
Pressure fluctuations, which normally occur in the exhaust system, may cause the exhaust gas to carry some cooling water upstream and into the cylinders. The backflow water tends to evaporate in a cylinder which has a higher operating temperature that the other cylinder. Evaporation can prove problematic where precipitates (e.g., salt) remain and cylinder. Under server condition, salt can corrode the piston and even lead to seizure of the piston within the cylinder.
A need therefore exists for a way of maintaining a generally uniform temperature between the cylinders even where exhaust gas flow differs between the cylinders.
An aspect of the present invention involves a multi-cylinder engine for a small watercraft. The watercraft comprises a first variable-volume combustion chamber and a second variable-volume combustion chamber. An exhaust manifold communicates with both the first and second combustion chamber. Means are provided for reducing the temperature of first variable-volume combustion chamber in comparison to the second variable-volume combustion chamber.
In accordance with another aspect of the present invention, a multi-cylinder engine for a small watercraft is provided. The engine comprises a first variable-volume combustion chamber and a second variable-volume combustion chamber. An exhaust manifold communicates with both the first and second combustion chambers. The first variable-volume combustion chamber exhausts into the exhaust manifold upstream of the point where the second variable-volume combustion chamber exhausts into the exhaust manifold. A cooling system includes at least a first coolant passage near the first variable-volume combustion chamber and a second coolant passage near the second variable-volume combustion chamber. A flow regulator of the cooling system produces a larger coolant flow rate through the first coolant passage than through the second coolant passage.
A preferred method of maintaining generally equal temperatures within at least two cylinders of a multi-cylinder engine involves cyclically providing a fuel/air charge to first and second cylinders of the engine and burning the fuel/air charge in the first cylinder and in the second cylinder. Combustion byproducts are ported to an exhaust manifold such that the first cylinder exhausts upstream of the second cylinder. The operating temperature in the first cylinder is reduced to generally match or to be lower than the temperature of the second cylinder during the cyclic operation of the a engine.